1. Field of the General Inventive Concept
The embodiments relate to a sawtooth wave generating circuit, and more particularly to a switch mode power supply device having the sawtooth wave generating circuit.
2. Description of the Related Art
Generally, a power supply device that supplies a power source is required to operate electronic devices. Recently a switch mode power supply (SMPS) device that supplies a stable DC output voltage is used in electronic devices.
The SMPS device converts a DC input voltage into stable output voltages that have various values by controlling a current through an inductor, using semiconductor devices as switching devices. The SMPS device has high power conversion efficiency, small size and light weight compared with a conventional linear power supply device because the SMPS device controls the switching of the semiconductor devices.
The SMPS device compares a feedback signal that is proportional to an output signal of the SMPS device and a sawtooth wave to generate a pulse-width modulated (PWM) clock signal. A sawtooth wave generating circuit included in the SMPS device generates a sawtooth signal in the process of charging and discharging a capacitor.
Conventionally, the slope of a sawtooth wave signal that is generated by a sawtooth wave generating circuit has been determined by charging current and capacitance of a capacitor, and the slope of the sawtooth has been designed to have a fixed value.
However, the charging current and the capacitance of the capacitor may be changed according to an operating condition such as a fabrication process, an operating voltage and an operating temperature. Further, a parasitic capacitance that is in the sawtooth wave generating circuit may be changed according to different operating conditions such as the fabrication process, the operating voltage and the operating temperature. Therefore, conventionally it was difficult to generate a sawtooth wave signal having a desired slope.
If the slope of a sawtooth wave signal is fixed, a waveform of the sawtooth wave signal may be distorted when a frequency of the sawtooth wave signal is very large, and amplitude of the sawtooth wave signal may be too small when a frequency of the sawtooth wave signal is very small.